This invention relates to a DC resistance welding machine incorporating an inverter and, more particularly to a weld power control system for such welding machine assuring a high and reliable quality of weld.
Recently, DC resistance welding machines that utilize an invertor in its power circuit have been developed and are spreading in the market.
As compared with conventional single phase AC resistance welding machine, the invertor type DC welding machine has the following advantages.
First, it is possible to reduce the size of a welding transformer incorporated in the inverter type welding machine since a high-frequency power is passed through the transformer. In applying the welding machine to a welding robot, such small sized transformer can be mounted on the end portion of an arm of the robot with no cable or electrical wire being necessary between the secondary coil of the transformer and a welding gun, thereby saving both cost and power consumption.
Secondly, DC welding by the inverter type welding machine is of higher heating efficiency, and needs a smaller weld current or a shorter duration of current flow or conduction in workpieces as compared with AC welding by the AC welding machine, thus, it is economical in power consumption and the life of electrodes can be elongated.
Thirdly, commercial three phase AC can be utilized as the power source of the inverter type welding machine, in which three phase balanced load and high power factor are obtained.
Heretofore proposed inverter type DC welding machines generally incorporate a constant-current controller or timer which functions to maintain constant the RMS value of weld current flowing during a welding operation. The controller or the timer is based on the idea of a constant-current phase control system of the single phase AC type welding machine and the weld current is deemed as the principal factor to obtain stable and high quality weld.
However, the weld current is DC in the inverter type welding machine while AC in the single phase AC type welding machine, and when such DC weld current is maintained constant, there arises some disadvantages. In case where workpieces to be welded are small metal assemblies or thin sheet of metal, for example, the resistance of the portion to be welded of the workpieces being located between electrodes varies rapidly during very short duration of welding operation such as 10.about.20 msec, and the contact resistance between the electrodes and workpieces in the beginning of the welding operation is relatively high due to a low pressure being appilied between the electrodes. In maintaining the weld current constant despite of such high contact resistance condition, it sometimes makes the resistance heat generation excessive which may be cause splashing or explosion of the melted workpieces. Furthermore, when the weld current is maintained constant even though the whole resistance between the electrodes has become decreased due to increase of the contact surface area between the electrodes and the workpieces in the last stage of the welding operation, it is sometimes results in that the the current density in the portion to be welded is too low to generate a sufficientquantity of resistance heat for obtaining a good weld.